1. Field of the Invention
The invention relates to a network, and more particularly to the service VLAN space of a provider network.
2. Description of the Related Art
Unlike a physical LAN, a Virtual LAN (VLAN) is a logically-independent network. The IEEE 802.1Q standard provides a tagging protocol, which appends a VLAN tag to the packet header to indicate the presence of VLAN information. The tagging protocol allows the VLAN to come into practice. Ordinarily, a VLAN tag is used to mark a logical subgroup or VLAN to which a packet belongs within a physical LAN, and the VLAN is thus established. This kind of VLAN tag is called a customer VLAN tag.
An Internet Service provider (ISP) is an organization, which provides Internet access to customers, and the network of the ISP is called a provider network. Similarly, with regard to the ISP, the VLAN tag is used to mark a logical subgroup to which a packet belongs within the provider network, and the packet can be rapidly forwarded according to the VLAN tag appended by the provider network. This kind of VLAN tag is called a service VLAN tag. The service VLAN tag and the customer VLAN tag can coexist in the provider network without interference.
FIG. 1 is an Ethernet frame format 100 including a VLAN tag conforming to the IEEE 802.1Q standard. The Ethernet frame 100 includes two Media Access Control (MAC) address fields 102 and 104, a VLAN tag field 106, a type/length field 108, a data field 110, and a cyclic redundancy check field 112. The length of both the destination address field 102 and the source address field 104 are 6 bytes, and respectively stores the destination and source MAC addresses. A 4-byte VLAN tag field 106 is inserted between the source address field 104 and the type/length field 108, storing information about the VLAN to which the frame 100 belongs. The type/length field 108 is followed by a data field 110, which is about 46˜1500 bytes in length and carries the user data. The CRC field 112 is 4 bytes in length and stores the checksum of the data.
The VLAN tag field 106 includes a tag protocol identifier (TPID) field 122 and a tag control information (TCI) field 124. The TPID field 122 is 16 bits in length and stores a TPID indicating that a VLAN tag header follows. The TCI field 124 further includes a user priority field 126, a canonical format indicator (CFI) field 128, and a VLAN identifier field 130. The user priority field 126 is 3 bits in length and stores the priority level of the Ethernet frame 100. The CFI field 128 is 1 bit in length. The VLAN identifier field 130 is 12 bits in length and stores a VLAN identifier indicating the VLAN to which the Ethernet frame 100 belongs.
A service VLAN space is the number of assignable service VLAN identifiers assigned by an ISP to customer equipment. The greater the number of customers accessing the Internet through the provider network, the larger the service VLAN space, the more flexibility the ISP has to group the packets, and the more efficiently the provider network functions due to the provider network forwarding the packets according to the service VLAN tag. Under the current IEEE 802.1Q standard, however, the service VLAN space contains only 4096 (i.e. 212) identifiers because the VLAN identifier field 130 of the VLAN tag header 106 is only 12 bits in length. This means an ISP can allocate a maximum of 4096 service VLAN identifiers to customer equipment, but the amount of customer equipment connected to the provider network of an ISP is far greater than 4096. Therefore, a method for expanding the service VLAN space of a provider network is desirable.